Q20 In 1981 children in the United States spent an average of slightly less than two and a half hours a week doing household chores; by 1997 they had spent nearly six hours a week. A. chores; by 1997 they had spent nearly six hours a week B. chores; by 1997 that figure had grown to nearly six hours a week C. chores, whereas nearly six hours a week were spent in 1997 D. chores, compared with a figure of nearly six hours a week in 1997 E. chores, that figure growing to nearly six hours a week in 1997
The answer is B. At first glance, I thought the B was right. But later I found "had grown" doesn't make sense. So i chose D. in the original sentence, "in 1981" use "spent", but why "in 1997" uses "had grown"? I am lost. Can sb. give me some directions? | 
